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How and why cutting onions makes us cry is one of life’s big mysteries

Donna Coutts, May 21, 2019 6:45PM Kids News

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Cutting an onion isn’t much fun when your eyes sting and the tears flow. media_cameraCutting an onion isn’t much fun when your eyes sting and the tears flow.


Reading level: green

Have you heard the joke about why onions make us cry?

Answer: Because they can’t crack a joke.

Anyone who has cut up an onion and felt tears running down their cheeks will know how good they are at making us cry.

But how do they do it? The Kids News team has investigated and has all the answers.

media_cameraCutting up an onion is enough to make you cry, even if you’re perfectly happy and looking forward to the dinner you’re cooking.

Each tiny cell* of an onion has a sealed blob within it containing a chemical the onion makes. This chemical is called an enzyme. Enzymes are very useful, natural substances in all living things that make chemical reactions happen.

When you cut an onion you are tearing the sealed blobs containing enzymes apart. The enzymes escape.

Once they have escaped they start reacting with another substance naturally in the onion, called sulfoxide.

The enzymes and the sulfoxide react to make a new substance called sulfenic acid.

This sets off a chain of other reactions one after another. The result at the end of this chain of reactions is another new substance called syn-propanethial S-oxide.

You don’t need to remember the name syn-propanethial S-oxide but you do need to know that this substance is lighter than air, so it floats upwards.

When you’re cutting an onion you have your head over the top of the chopping board. Up the syn-propanethial S-oxide floats and into your eyes.

Once it’s there, it reacts in another chemical reaction with your tears, making tiny amounts of yet another new substance called sulfuric acid.

The brain recognises the danger of having sulfuric acid near your eyes and tells the glands* around your eyes to start making heaps of tears.

The tears wash the sulfuric acid out of your eyes.

media_cameraOnions are so good at making us cry some people will try anything to stop it happening.

In the wild, onions are perennial* bulbs, which mean they keep growing in the ground for more than one year. The strong chemicals that make us cry are to stop animals, including insects, eating them when they’re growing in the ground.

Humans have been trying to beat this onion-cutting-crying problem since we decided they were a good thing to cut up and eat.

There are thousands of so-called solutions.

These include sucking on a spoon, keeping a piece of bread in your mouth and standing on one leg while singing the national anthem. (Actually, we just made that one up.)

Ideas that could work include wearing swimming goggles, freezing the onion, running it under cold water and using a super sharp knife (with an adult supervising).


  1. What’s the difference between a viola and an onion? No one cries when you cut up a viola.
  2. What do you call a hobbit with a healthy appetite? Lord of the Onion Rings.
  3. Why does Mr Potato Head have a mobile phone? In case Mr Onion Rings.
  4. What did the carrot say to the onion? Nothing, vegetables don’t talk.
  5. Not sure if I’m crying because I don’t get the onion jokes or because I’m cutting up an onion.

VIDEO: This big brother tried to play a trick but it backfired, because the little boy really loved the raw onion!


  • cell: small compartment in a living thing; the smallest unit of life
  • glands: groups of cells in animals that makes substances we need, such as tears, sweat and hormones
  • perennial: opposite of annual; lives for more than one year


What is a hiccup?

Why do we need sleep?

Beware of wild mushrooms

Scientists discover why zebras have stripes

Why is our blood red?


  1. What is an enzyme?
  2. Where do tears come from in our eyes?
  3. What is the chemical that hits our eyes and causes us to cry?
  4. Why have onions evolved to have this effect on us?
  5. Name two possible ways to avoid crying while cutting an onion.


1. Brain Power
One of the most interesting things in the scientific process between cutting and onion and crying is that the human brain tells the eyes to release tears to get rid of a harmful chemical.

Can you think of some other things that the brain does to help protect us from harm? Work with a partner and fill out a table with two columns. Label the first column POTENTIAL HARM (for example, onion releasing sulfuric acid when reaching the eyes). Label the other column BRAIN REACTION (for example, glands around eyes produce lots of tears to rid the eyes of the harmful chemical).

Time: allow 15 minutes to complete this activity
Curriculum Links: English, Science, Critical and Creative Thinking, Personal and Social.

2. Extension
Create two more onion jokes of your own. If you are comfortable, share them with the class and see whose joke gets the most laughs!

Time: allow 10 minutes to complete this activity
Curriculum Links: English, Critical and Creative Thinking

With a partner see if you can identify all the doing words/verbs in this text. Highlight them in yellow and then make a list of them all down your page. Now see if you and your partner can come up with a synonym for the chosen verb. Make sure it still makes sense in the context it was taken from.

Try to replace some of the original verbs with your synonyms and discuss if any are better and why.

HAVE YOUR SAY: Do onions make you cry? Do you have a solution you think could work?
No one-word answers. Use full sentences to explain your thinking. No comments will be published until approved by editors.

Extra Reading in explainers